Conventional methods of mounting bolts, for example to strengthen the roof and walls of rock tunnels, do not in themselves offer a guarantee that the bolts are satisfactorily anchored in the rock. It is not possible to visually determine whether, for example, a concrete-bonded bolt has the prescribed length or that the bond is satisfactory, i.e. it has the desired load-carrying or load-transmitting abilities. Further it is possible for a bolt which was originally mounted correctly and which initially was in good contact with the surrounding rock and was sufficiently load-carrying, to subsequently become loose and at least loose a considerable part of its load-carrying ability. This may occur, for example, as a result of the bolt being subjected to shear forces or tension forces as a result of movement in the surrounding rock. If the bolt should fracture, it is impossible to detect the fracture visually, even though the fracture should occur at only a very short distance from the outer surface of the concrete-bond. There is thus a need for methods and means of checking the length and the function of bolts.
One method of checking the anchorage of a bolt and its load-carrying ability is to apply a tension force to the bolt, by means of a hydraulic jack for example, until the bolt fractures or loosens. Because of the costs involved and the amount of time required, normally only a small percentage of the total number of bolts can be checked. Furthermore, the load-carrying ability of the bolts and the corrosion-protective effect of the concrete bond can be impaired by subjecting them to tensile tests, rendering the tested bolt unserviceable, even though the bolt has not been pulled until it loosens or fractures. Furthermore, such tensile tests have a limited value, since it has been established that a bonding length of about 30 cm is sufficient to hold the bolt such as to cause the bolt to fracture when a tensile load is applied thereto. Thus, a tensile test in which the end of the bolt fractures only shows that the bolt had a satisfactory bond length of at least approximately 30 cm.
The object of the present invention is to provide means in which the length of rod-like elements, such as rock bolts and the like, and the anchorage conditions thereof can be nondestructively investigated in-situ without causing damage to said elements or impairing said anchorage. Thus, a rock bolt or like element subsequent to being tested shall be capable of being used for reinforcing purposes or for load-carrying purposes or for other purposes. By subjecting all, or a sufficient number of selected bolts within a given limited area to such a non-destructive examination, it is possible to establish whether the reinforcement of a wall or a roof structure is sufficient, with respect to prescribed safety requirements. The non-destructive examination is made with the help of elastic oscillations.
It has long been known that elastic oscillations of a multiplicity of different wave types, can, under certain conditions, propagate along a circular-cylindrical homogenous body. Examples of such wave types include longitudinal waves, torsional waves, radial waves and flexural waves. When the oscillations have a sufficiently high frequency it is generally possible for more than one oscillation mode of respective wave types to propagate along the circular-cylindrical body. If, for the sake of simplicity, one limits oneself to the simplest oscillation mode of respective wave type and the lowest frequency thereof, it is relatively simple to describe the different wave types in a manner such that their differences are clearly apparent. The oscillation mode of the longitudinal wave is characterized by the fact that the entire cross section of the body is alternately compressed and expanded in the longitudinal direction thereof. The oscillation mode of the radial wave is characterized by the fact that the entire cross section of the body is alternately compressed and expanded in the radial direction. The oscillation mode of the torsion wave is characterized by the fact that adjacent cross sections of the body twist relative to each other around the axis of the body. The oscillation mode of the flexural wave is characterized by the fact that certain parts of a cross-section of the body expand in the longitudinal direction thereof at the same time as other parts of said cross-section are compressed in the said longitudial direction. The said parts are separated by a diametrical, neutral plane located parallel with the sense of propagation of the oscillation mode, i.e. with the longitudinal axis of the circular-cylindrical body.
For a more exhaustive description of elastic waves in rod-like bodies, reference is made to the article "Elastic Waves in Rods and Clad Rods", by R. N. Thurston, published in the Journal of Acoustical Society of America, 64 (1), July 1978.
The propagation of waves through a concrete-bonded bolt is, for a number of reasons, more complicated to describe theoretically than the wave propagation in a circular-cylindrical homogenous free body. One reason, of course, is because the bolt is bonded in concrete and consequently not free. The contact of the concrete bond with the outer surfaces of the bolt causes certain restrictions to the possible compression and expansion of the cross-section of the bolt, at least in those parts of said cross-section lying closest to the said outer surface of the bolt. Another reason is that bolts do not normally have the form of a circular-cylindrical body. The majority of bolts today comprise reinforcing rods provided with a multiplicity of shoulders or teeth along their outer surfaces.
The shoulders extend either tangentially at right angles to the longitudinal axis of the bolt, or at an oblique angle to said axis. Neither do the shoulders extend completely around the circumference of the bolt, but that certain bolts have peripheral portions which are not provided with such shoulders. In certain cases, the peripheral surface of the bolts may also be provided with one or two shoulders which extend in the longitudinal direction of the bolt. The result is that the cross-sectional shape of the bolt is neither circular nor constant therealong, but varies substantially along the length of said bolt.
For a more exhaustive description of wave propagation in rod-like elements which may vary in cross-section along the length thereof, reference is made to the article "Wave Propagation in Non-uniform Elastic Rods" by Gerald Rosenfeld and Joseph B. Keller, published in the Journal of the Acoustical Society of America, volume 57, number 5, May 1975, pages 1094-1096.